(Anaerobic formulations, i.e. (meth)acrylic ester formulations which are oxygen stabilized and cure as a result of oxygen deprivation,) have become well known since their disclosure in U.S. Pat. No. 2,628,178. The invention of formulations employing hydroperoxide catalysts which are stable without aeration which was disclosed in U.S. Pat. No. 2,895,950 (Krieble) prompted a wide range of commercial uses for such formulations including thread locking, impregnation of porous articles, gasketing and adhesive bonding applications. Acceleration of the cure of such composition has been disclosed in numerous patents. (Sulfimide accelerators, particularly saccharin (benzoic sulfimide),) have been used in most anaerobic formulations since the disclosure of their utility in U.S. Pat. No. 3,046,262. Other accelerator systems which are known for use in anaerobic formulations include various amine compounds as disclosed, for instance, in U.S. Pat. Nos. 3,041,322, 3,203,941 and 3,218,305 and various compounds having structures analogous to sulfimides such as disclosed in U.S. Pat. No. 4,513,127 and U.S. Pat. No. 4,622,348 and the references disclosed therein. Certain acyl hydrazine compounds are disclosed as anaerobic accelerators in U.S. Pat. Nos. 4,180,640 and 4,287,330preferably in combination with an acid having a small pKa of 6 or less. The various known types of accelerators are often used in combination in commercial formulations.
Similar acrylic curable peroxy initiated compositions which rely on chemical activation rather than oxygen deprivation to activate cure initiation are disclosed, for instance, in U.S. Pat. Nos. 4,442,267; 4,450,030; 4,451,615 and 4,574,142. Preferred activators for such compositions are amine/aldehyde condensation products.
Formulations employing combinations of anaerobic and chemical activation are also known. For instance, in U.S. Pat. No. 3,672,942 there is described an impregnation process employing the steps of impregnating a porous article with an anaerobically curable composition and then treating the surface with a chemical activator to induce surface cure where the impregnant is exposed to oxygen. Suitable activators include solutions of amine/aldehyde condensation products, various metal salts or chelates and various thiourea or other sulfur containing compounds. While the presence of uncured surface monomer has been reduced by the improved processes of U.S. Pat. No. 3,969,552 and U.S. Pat. No. 4,165,400, it is still generally advised to use an accelerator rinse to activate cure of the composition nearest of the surface of the impregnated article. Erythrobic or ascorbic acids have become preferred impregnation surface activators in more recent years.
It has also (long been known) that transition metals, especially copper or iron, play an important part in the cure mechanism of peroxide initiated acrylic formulations. This is especially so in anaerobic cure mechanisms. Transition metal oxidation is often part of the redox reaction which produces free radicals from peroxy initiators. Transition metal may be supplied by the substrate itself or in the form of salts of metals incorporated into the formulation or applied to the substrate. Copper, manganese, iron, cobalt and vanadium salts are all known for this purpose, copper and iron being the most commonly used. In impregnation processes which employ aeration of the acrylic monomer formulation it is typical to add 1-10 ppm copper as a copper salt directly to the formulation. As disclosed in U.S. Pat. No. 4,632,945, combinations of copper and iron salts or complexes may also be employed.
Recently it has been discovered that certain anaerobic formulations as described in U.S. Pat. Nos. 4,180,640 and 4,287,330 will not readily cure on metal substrates treated with nitrite, silicate, borate or alkaline carbonate corrosion inhibitors even though these compounds are typically present on the surface at a level of only a few ppm. Cure inhibition by nitrite salts was observed to be most severe. This discovery prompted an investigation of the effect of such salts on the cure of anaerobic compositions.